1. Field of the Invention
The present invention relates generally to the field of data communications. More particularly, the invention relates to methods and apparatus for wirelessly transferring data from a display screen to an external receiving device.
2. Background
A number of techniques have been proposed for transferring data from the display screen of a computer system to an external device such as a personal digital assistant (PDA) or other portable information device. Most typically, the prior art techniques modulate the video signal applied to a cathode ray tube (CRT) display. The modulated video signal is then detected and decoded using a photo detector and microprocessor in a receiving device.
U.S. Pat. No. 4,999,617 issued to Uemura, et al. discloses a device for reading patterns displayed on a CRT display unit of a television receiver. A bar code pattern is displayed within a small region of the CRT display unit. A photodetector is attached to the surface of the CRT display unit with a suction cup over the region in which the bar code pattern is displayed.
U.S. Pat. No. 5,488,571 issued to Jacobs, et al. discloses a method and apparatus for downloading information from a CRT display to a portable information device. Binary coded data are formatted into blocks of serial data bits, which are displayed as a pattern of spaced horizontal lines. Essentially the same system is also the subject of U.S. Pat. No. 5,535,147 issued to Jacobs, et al. and U.S. Pat. No. 5,570,297 issued to Brzezinski, et al.
U.S. Pat. No. 5,231,488 issued to Mohrbacher, et al. discloses a system for displaying and reading patterns displayed on a CRT display unit. A photodetector is attached to the display screen with a suction cup in the manner disclosed by Uemura, et al. The receiving apparatus detects an increase in intensity of a synchronizing scan line and an increase in intensity of a data scan line and then utilizes the time differential between the two events to determine the location of the increasing intensity of the data scan line. This location is determinative of the value of the data scan line.
U.S. Pat. No. 5,652,602 issued to Fishman, et al. discloses another serial data transmission technique using a CRT display of a computer. The computer is programmed to display sequential frames containing illuminated line segments that represent individual binary data bits. Each bit is encoded as a different line segment length to produce an optical pulse having a duration dependent on the value of the data bit. Thus, a "0" bit is represented by a pulse with a duration relatively longer than that of a pulse representing a "1" bit. Essentially the same system is also the subject of U.S. Pat. No. 5,748,895 issued to Shiff, et al.
All of the above-mentioned patents are specifically adapted for transferring data from a CRT display. CRT displays use a scanning electron beam to excite the phosphor coating on the face of the screen. As the beam scans across the screen it causes the phosphor coating to emit light. Once the beam has passed by, the phosphor continues to emit light but the brightness quickly decays over time. A photodetector aimed at the face of a CRT display will detect the fast rise of the phosphor luminance as the electron beam scans across the screen. The phosphor luminance will then decay until the next scan of the screen. In the CRT, the beam basically paints the image one dot (pixel) at a time.
Flat panel displays are rapidly becoming popular alternatives to CRT displays, not only in "notebook" computer systems, but in desktop systems as well. A variety of flat panel displays have been developed and proposed, including LCD, electroluminescent, plasma and others. The flat panel displays in common use today typically comprise a liquid crystal material placed in front of a fluorescent tube. The fluorescent tube is the actual source of the light in these types of LCD panels. Light from the fluorescent tube passes through polarizing filters, color filters and liquid crystal materials before radiating from the front of the screen. The liquid crystal material acts as a shutter to selectively block or pass light from the fluorescent tube, on a pixel by pixel basis.
The liquid crystal material is relatively slow to respond in comparison to a scanning CRT tube. FIG. 1 illustrates the relative luminance signals detected from CRT and LCD displays. Flat panel displays produce an electrical field across the liquid crystal to keep the crystals aligned to allow light to pass through. These drive signals are produced a line at a time. This means that one entire line on the display is driven at a time, with each pixel in that line being driven either on or off. After driving one line for a period of time, the driving electronics advances to the next line until all lines have been driven. A photodetector aimed at the front of a LCD panel will not detect any significant luminance change due to this scanning. This is because the liquid crystal is slow to respond and designed to hold its on or off state until the next display field. Consequently, data transfer methods adapted for use with CRT displays, such as those disclosed in the prior art patents mentioned above, are unlikely to function properly with an LCD display. At the very least, the data transfer rate will be significantly reduced.
The two types of flat panel LCD displays most widely used today are passive and active matrix. Both of these types work in a similar manner, but the active matrix provides a brighter picture compared to the passive matrix. Passive matrix panels have a matrix of electrodes which can be selected to drive the liquid crystal for each video pixel. The matrix is driven with a multiplex signal which commonly activates one line of the display at a time. This line scanning provides a periodic electrical bias to keep the liquid crystals oriented to either pass or block light from the fluorescent tube. The liquid crystals can not switch on and off at the line multiplexing rate. In a passive display the crystals take a very long time to transition between ON and OFF states. Passive panels typically require up to 300 msec. to respond (1/3 of a second).
Passive panels can also be dual scan (DSTN) in which the screen is divided into two separate scanning sections. This means that at any given time, two separate areas of the screen are being driven by the drive electronics.
Active matrix displays are similar to passive displays except that they have a transistor located at each video pixel. The transistors are selected a line at a time and each transistor in the line is either turned on or off. Note that even though the transistors are selected on a line by line basis, they have a storage element that holds the drive signal on the liquid crystal, even after the line drive signal has passed. This provides a better bias signal for the liquid crystal and allows more light to pass through from the fluorescent tube. This active approach allows for a longer drive potential at each pixel and produces better brightness and display quality compared to the simpler passive matrix display. Active panels typically require up to 50 msec. to respond (1/20 of a second). While this is much faster than passive panels, it is slower than CRTs. CRT displays respond very quickly with frame rates as high as 13 msec. (1/75 of a second).
Most current approaches to video modulation data transfer use sequential pulsing of the video image to provide a series of binary 1's and 0's. These binary bits are used with framing bits (start and stop bits) to form complete data bytes. Some of the current approaches rely on the scanning CRT image to serialize the data bits by providing a luminance pulse for each data bit This approach will fail when applied to flat panel LCD screens because these screens do not have a scanning luminance response like that found with the CRT.
Other methods provide a binary bit stream where each bit is produced at the video field rate. For a typical CRT, this provides one binary data bit each 16 msec. (60 fields per second). While this approach is viable for the CRT, it will not work well for flat panel displays. The slow response time of LCD panels mean that only a small number of data bits could be transferred per second. For a passive display, 3 bits would be possible (assuming 300 msec. response time). For active panels, 20 bits could be transferred. Using conventional start and stop bits, a passive panel would then be capable of transmitting 0.3 bytes per second and active panels 2 bytes per second. This is too low a data rate for many applications.
U.S. Pat. No. 5,347,110 issued to Audebert, et al. discloses a system for transmitting information by the time modulation of a character display area of an LCD or similar display. Binary information is transmitted by the binary modulation of the reflectance of an area of the LCD display. A reader device includes one or more light sources arranged to illuminate the modulated area of the LCD display. A photosensor is used to detect the relative reflectance of the display area.